Advent of Code 2021

All my Advent of Code repos:

• AoC 2015 in Nim, Python
• AoC 2016 in Python, Clojure (+ visualizations)
• AoC 2017 in Nim, OCaml, Python
• AoC 2018 in Nim, Python, Racket
• AoC 2019 in OCaml, Python, Clojure
• AoC 2020 in Nim, one liner-y Python, Racket
• AoC 2021 in Python, Racket (this repo)
• AoC 2022 in Python, Clojure
• AoC 2023 in Clojure
• AoC 2024 in Clojure (Clerk notebooks), Python, Elixir
• AoC 2025 in Clojure (Clerk notebooks)

This year it is back to roots - Python!

I'll probably add solutions in other languages on some later date.

EDIT, November 2022: And I did - I've solved initial days in Racket too.

EDIT, November 2025: Days 19, 22, 23 were solved in Clojure.

Solutions

Task | Python Solution | Racket Solution | Comment for Python solution --- | --- | --- | --- Day 00: Helper file | aoc.py | aoc.rkt | Utilities to use for solving the tasks. Day 01: Sonar Sweep | day01.py | day01.rkt | (a + b + c) < (b + c + d) --> a < d Day 02: Dive! | day02.py | day02.rkt | "Up, up, down, down, left, right, left, right..." Day 03: Binary Diagnostic | day03.py | day03.rkt | from collections import Counter Day 04: Giant Squid | day04.py | day04.rkt | Set operations (<=, -) to the rescue. Day 05: Hydrothermal Venture | day05.py | day05.rkt | TIL defaultdict is faster than Counter. Day 06: Lanternfish | day06.py | day06.rkt | Recursion with memoization. Day 07: The Treachery of Whales | day07.py | day07.rkt | Mean mean! Day 08: Seven Segment Search | day08.py | day08.rkt | Using Python 3.10 and pattern matching. Day 09: Smoke Basin | day09.py | day09.rkt | Recursive DFS. Day 10: Syntax Scoring | day10.py | day10.rkt | dict(zip(...)) for easier typing Day 11: Dumbo Octopus | day11.py | day11.rkt | Iterative DFS. Day 12: Passage Pathing | day12.py | day12.rkt | Recursive DFS generator with yield from. Day 13: Transparent Origami | day13.py | day13.rkt | Fold. Not fold. Day 14: Extended Polymerization | day14.py | | Counter to the rescue. Day 15: Chiton | day15.py | | Dijkstra. Day 16: Packet Decoder | day16.py | | iter for some elegant stream implementation. Day 17: Trick Shot | day17.py | | Analytical + brute force. Day 18: Snailfish | day18.py | | My first ever eval in 7 instances of AoC. Day 19: Beacon Scanner | day19.py | day19.clj | Day 20: Trench Map | day20.py | | I had 9-neighbours helper ready. Day 21: Dirac Dice | day21.py | | Lanternfish says hi! Day 22: Reactor Reboot | day22.py | day22.clj | Day 23: Amphipod | day23.py | day23.clj | Day 24: Arithmetic Logic Unit | day24.py | | Day 25: Sea Cucumber | day25.py | |